Non-positive-displacement machines such as, for example, steam or gas turbines, are employed as heat engines in engineering in order to convert an energy stored in a gas flow into a mechanical energy, in particular into a rotary motion. In order to increase the efficiency of such machines, increasingly high physical demands are being placed on the materials used in the non-positive-displacement machine. So, to increase the efficiency of a gas turbine, the temperature of a gas flow flowing into the gas turbine is raised to more than 1200° C. In order to be able to withstand the high physical demands, in particular due to the temperature, the blades of the turbine are equipped with a coating which will withstand a particularly high loading. Such a coating is, for example, the thermal barrier coating of blades of a gas turbine, hereinafter referred to as TBC, a blade being furnished on its surface exposed to the gas flow with a coating of this type. Yttrium-stabilized zirconium oxide, for example, can be considered as a coating. Due to the temperature-dependence of the efficiency, it is desirable for the temperature of the gas flow to be increased up to a maximum threshold value without the TBC being damaged. For this purpose, measurement methods and devices are required for determining the exact temperature of the blade surfaces and in particular of the blade surface coatings during operation of the non-positive-displacement machine.
A common option for determining the temperature is the attachment of thermoelements to the points to be examined. Due to the high temperatures in the gas flow of the non-positive-displacement machine, the thermoelements generally fail after a few hours of operating time and are in most cases destroyed.
A contactless alternative to thermoelements is presented by optical pyrometers, by means of which the temperature of the object can be inferred in an optical manner from the radiation radiated by the hot object under examination. A corresponding pyrometer is indicated in GB 2 109 472 A. Since the radiation pattern and thus the emission coefficient of TBC has been studied relatively little and is consequently not known precisely, only a relatively imprecise temperature measurement can be undertaken herewith.